This invention relates to a method of forming electrodes on multi-layered epitaxial structures, and more particularly to forming electrodes on an electro-optic tunable filter.
Optical bandpass filters whose transmission characteristics may be controlled by the application of DC voltages are useful in a variety of different applications, such as infrared focal plane technology, optical signal processing, high resolution spectroscopy, solar physics, astrophysics, and high pressure tunable lasers.
Some examples of the electro-optic devices are disclosed in U.S. Pat. Nos. 3,164,665 to Stello; 3,740,512 to Siegel; 3,741,626 to Wentz; 4,240,696 to Tracy et al; and 4,269,481 to Yeh et al.
One proposed and highly attractive configuration for an electro-optic tunable filter consists of a stack of epitaxially grown layers of alternating high and low resistivity. The low-.rho. layers, which must be transparent and are thus very difficult to distinguish from the high resistivity layers, act as field controlling electrodes. They must be individually and independently provided with a reliable electrical contact. Because these layers are quite thin (one micron or less), and are only separated from each other by about 20-25 microns of high resistivity material, contact to any one layer by conventional means involving an opening (generated by a preexisting mask set) in an insulating layer is subject to serious alignment difficulties. The thin layers are difficult to see in cross-section and their actual location is determined by manufacturing tolerances in the liquid phase epitaxial (LEP) growth process. Any hole pattern generated by a mask would be required to use very small holes (approximately 10 microns in diameter, or less) in order not to run the risk of contacting the wrong layer because of layer thickness tolerance runout over a stack of, say, 50 layers. The possibility of missing a contact also exists for the same reason for a set of holes generated from a preexisting pattern.
To the best of my knowledge, prior to this invention no suitable means for locating and contacting these layers has been proposed which would allow for cumlative tolerance variations caused by slight fluctuations in layer thickness or position. Tracy et al and Yeh et al for example discuss extremely thin layers, but are silent as to formation of the electric contact. Some actual experimental filters may have used relatively thick layers, which would facilitate locating the electrodes.